The Yale study, which appears online in the Journal of Adolescent Health, showed that alcohol, cigarettes and marijuana were associated with an increased likelihood of prescription drug abuse in men 18 to 25. In women of that age, only marijuana use was linked with a higher likelihood of prescription drug abuse.

For years, researchers have looked at a connection between marijuana and hard drugs, such as cocaine and heroin, said Lynn Fiellin, the study's lead author and an associate professor of medicine at the Yale University School of Medicine. But given the large number of people who abuse prescription drugs -- particularly opioids (or painkillers) such as OxyContin and Percocet -- Fiellin said it seemed worthwhile to examine whether there was a link between marijuana and use of these drugs.

"I don't think the general population has a good idea of how serious the problem is with prescription opioids," Fiellin said. "When they're abused or misused, these are hard drugs."

In their research, Fiellin and her team looked at data from the 2006, 2007 and 2008 versions of the National Survey on Drug Use and Health, as those were the most recent surveys available at the time of their study. The Yale researchers focused on a sample of 55,215 18- to 25-year-olds. Of those, 6,496, about 12 percent, reported that they were abusing prescription opioids. Of the group abusing these drugs, about 57 percent had used alcohol, 56 percent had smoked cigarettes and 34 percent had used marijuana.

The study found that, among both men and women, those who had used marijuana were 2.5 times more likely than those their age who abstained to later dabble in prescription drugs. Also, young men who drank alcohol or smoked cigarettes were 25 percent more likely to abuse prescription opioids. However, the study didn't show an association between alcohol or cigarette use in young women and later use of prescription drugs.

Erik Williams, executive director of the Connecticut chapter of the National Organization for the Reform of Marijuana Laws, said the study fails to make a strong link between marijuana and the use of harder drugs. NORML is a nonprofit, public-interest lobby that supports the removal of criminal penalties for possession and use of marijuana by adults.

"This is just another propaganda study that tries to turn a casual relationship into a causal relationship," Williams said. "There's no real conclusive evidence here."

Fiellin conceded that more research is needed to prove a concrete connection between opioid abuse and marijuana, alcohol and cigarettes. However, Fiellin said, this study is a start.

"It's a red flag," she said. "It sort of highlights that there's a potential association that's important here."

At least one area mental health professional said the Yale findings don't come as a surprise. Susannah Tung, a staff psychiatrist at St. Vincent's Behavioral Health in Westport, said she works with people battling addiction and frequently sees how early use of alcohol, cigarettes and marijuana can open the door to harder substances.

"If you start using something, it easily and quickly worsens," Tung said.